The invention pertains to a process for the carburizing of workpieces of carburizable materials, especially steels, by means of a pulsed plasma discharge in a carbon-containing atmosphere at pressures of 0.1-30 mbars and at pulsed voltages of 200-2,000 V, preferably of 300-1,000 V.
In a process of this type known from EP 552 460 A1, the voltage at the electrodes during the so-called pauses between the pulses is zero, the electrodes consisting of at least one electrode on the machine side and the workpieces or the holder of the workpieces on the other side. That is, the process is operated without a so-called baseline voltage.
Not only ferrous materials but also nonferrous materials such as titanium are included among the materials which can be carburized.
When structural parts of steel are carburized in a pulsed glow discharge (plasma), an intense flow of carbon is created at the start of the carburizing operation, so that the carbon content at the edge of the structural component increases as rapidly as possible to values just below the saturation limit. As a result, the steepest possible carbon gradient is created in the component at the start of the treatment, which has positive effects on the properties of the finished products.
The flow of carbon depends on the parameters of the plasma. To generate a high carbon flow, the amount of power which is introduced into the plasma must be on a correspondingly high level. The electric current which develops in the plasma during a pulse depends on the surface area of the components to be treated and usually reaches orders of magnitude of 25 A/m.sup.2 of surface area. For the treatment of large batches, it is therefore necessary to use generators with pulse outputs of more than 200 A at voltages of 500-1,000 V. The corresponding outputs must be switched on an off at intervals in the range of about 10-100 .mu.s. Generators with outputs of this sort are not available on a production-line basis; these are expensive, custom-made machines.
It is known from DE-PS 601 847 that, when individual workpieces of metal are hardened by gas diffusion under additional heating and the action of a pulsed plasma, the duration of the pauses between the individual surge pulses should be selected so that the gas can undergo deionization; these intervals are usually at least ten times longer than the surge pulses themselves. This means that the ionization must be built up again each time from an energy level of zero. For example, the pulse frequency can be 10 Hz and the average current 100 mA.
When the workpieces are subjected to supplemental heating in the conventional manner, U.S. Pat. No. 4,490,190 informs us that, by means of an appropriately high frequency of short pulses with long pauses between them, it is possible to generate a cold plasma, which has the effect of disconnecting the heating action of the plasma from its thermochemical effect on the workpieces. As a result, it is possible to avoid thermal .damage to the workpieces. No measures for preserving some of the ionization during the pauses between the pulses are stated, however, it can be assumed that the treatment time is relatively long and/or that the penetration of the gases is relatively shallow. Neither the size of the workpieces, the size of the batch, the current density, nor the total current is stated.
The invention is therefore based on the task of generating higher carbon flows with the use of relatively small generators and thus to reduce the investment and operating costs of a system for implementing the process.